Detecting a damage of a wind turbine blade on the basis of strain arising in the wind turbine, is known.
For example, Patent Document 1 discloses an apparatus to detect a strain of each of a plurality of wind turbines by means of a strain gauge mounted on each of the wind turbines and to monitor abnormality of the wind turbine blades by using the detection results.
In this apparatus, the output of the strain gauge for each blade is converted into an electric signal proportional to the stress level, and as to a component representing a magnitude of variation of the stress, difference between two of the blades is calculated. Then, it is determined that there is an abnormality in any of the wind turbines when the calculated difference between the two of the blades excesses a prescribed value over a prescribed length of time period.